Field of the Invention
The present invention relates to a sheet processing apparatus that applies a post process to a sheet output from an image forming apparatus, and to an image forming system that is provided with the sheet processing apparatus.
Description of the Related Art
There is a known sheet processing apparatus that is arranged at a downstream side of an image forming apparatus, such as a copying machine and a printer, applies a post process, such as a binding process, to a sheet (paper) output from an image forming apparatus, and conveys and aligns a transparent sheet with high transmittance, such as an OHP sheet, or a translucent sheet.
When a sheet processing apparatus performs post processes including a binding process, an alignment member is operated in synchronization with conveyance of a sheet in order to raise quality of a result. For example, there is a known technique that corrects conveyance error for every sheet by operating an alignment member in synchronizing with a detection of a sheet end detected by a detection sensor that detects an end of a sheet conveyed by conveying rollers (Japanese Laid-Open Patent Publication (Kokai) No. H10-279166 (JP H10-279166A)).
Incidentally, there is an increasing demand to perform a printing process and a post process to a translucent sheet of which transmittance is high but is not as high as an OHP sheet in recent years.
Although an optical sensor with high detection accuracy, such as a transmission optical sensor, is employed as a conveyance sensor in order to detect a position of a conveyed sheet correctly, the transmission-optical sensor may not correctly detect a translucent sheet with high transmittance. In this case, since the sheet end is not detected even if predetermined time passes from a predetermined timing, a conveyance jam is erroneously determined, and the conveyance of sheet is stopped on the basis of the erroneous determination. This wastes the results and delays the process.